Electronic pens are known and may incorporate a positioning system which actively traces the movement of the pen while it is used for writing on a passive substrate, such as a piece of paper, plastic, carton etc. The positioning system may infer the movement from, for example, a roller ball contacting the substrate, triangulation of signals from one or more external transmitters, integrated acceleration sensors, strain sensors associated with a pen point contacting the substrate, images of a pattern on the substrate, or combinations thereof.
Applicant's WO 02/39377 discloses an electronic pen with an internal time circuit, by means of which each pen stroke can be time-stamped, i.e. associated with a particular time given by the time circuit, typically the time when the pen stroke was recorded by the positioning system or stored in pen memory. The timestamps may then be used, by processing circuitry in the pen or in a receiving unit, in processing of the pen strokes. The timestamps can, i.a., be used to identify any pen strokes that have been recorded since a transmission time point when pen strokes were last transmitted to the receiving unit. The timestamps may also be used to organize the pen strokes with respect to different objects, e.g. different notebooks. If pen strokes are first recorded from a first notebook, and a new notebook is then activated at a certain activation time point, whereupon pen strokes are recorded from this second notebook, the resulting pen strokes can be selectively associated with the first and second notebooks based upon the relation between the timestamps and the activation time point.
Applicant's WO 03/046708 discloses a similar electronic pen, in which each pen stroke is provided with a timestamp given by an internal time circuit. The timestamps are then used, by processing circuitry in the pen or in a receiving unit, to allow functions given by certain pen strokes to operate on pen strokes recorded immediately before or after such certain pen strokes.
Still further, Applicant's WO 03/042907 discloses a method for freeing memory in an electronic pen, in which pen strokes are selectively deleted from pen memory based upon their respective timestamp.
There is thus a desire to have a well-controlled time circuit in each electronic pen.
Applicant's WO 03/005181 discloses a communication protocol used in communication between electronic pens and network servers in an information management system. The pens typically connect to the servers via a network connection unit, such as a mobile phone or a personal computer. According to the communication protocol, a central network server (paper look-up server) is authorized/trusted to submit a time update value to the pen which, upon receipt of the time update value, adjusts its internal time circuit accordingly. Thereby, all pens in the information management system can have their time circuits set in synchronization with a common time reference given by the central network server. Consequently, pen strokes recorded with the pens can be time-stamped with respect to a well-controlled and common time reference.
However, the need for a well-controlled time circuit may run contrary to the demands of the pen user, who may want to be able to adjust the time reference in which the strokes are time-stamped. For example, the pen user may instead want to have the timestamps reflect the exact time of a local device, e.g. the network connection unit which connects the pen to the network. Clearly, this may cause problems if resulting timestamps are used in the processing of the recorded pen strokes.